1. Field of the Invention
This invention relates to a synchronization system for use in a radio base station of a cellular time division multiple access (TDMA) mobile communication network in controlling a transmission time instant of a down-link radio signal.
2. Description of the Related Art
Various digital mobile communication networks are recently known. In most of such networks, TDMA schemes are used. For example, a paper was presented at the 39th IEEE Vehicular Technology Conference held Apr. 29 to May 3, 1989, in San Fransisco, Calif., by Heinz Ochsner under the title of "DECT--Digital European Cordless Telecommunications".
In the manner which will later be described in greater detail, a general cellular mobile communication network has a service area divided into a plurality of cells or radio zones. A radio base station is situated in each radio zone. Such radio base stations are consequently in one-to-one correspondence to the radio zones. The radio base stations may be connected to a control station through wired and/or radio links. The control station is connected to a fixed communication network through wired links. At least one mobile station is present alive or active in the service area and may move from a radio zone to another radio zone. The mobile station, as herein called, may either be a cordless telephone set or a communication terminal installed on a mobile vehicle, such as an automobile.
While present active in a radio zone, a mobile station transmits an up-link radio signal to a radio base station corresponding to the radio zone under consideration. On the other hand, a radio base station transmits a down-link radio signal to mobile stations which may be currently present alive in a radio zone corresponding to the radio base station in question. Two adjacent ones of the radio base stations have an overlapping area in which a mobile station can receive the down-link radio signals from the radio base stations corresponding to the radio zones having the overlapping area and can transmit the up-link radio signal to these radio base stations.
When used in time division multiplex communication, each of the down-link and the up-link radio signals is represented by a sequence of time slots. The down-link radio signal comprises a down-link communication data signal and a down-link control signal which is used to indicate the mobile stations present in the radio zone corresponding to the radio base station transmitting the down-link radio signal in question. Incidentally, each radio base station can judge whether a received up-link radio signal is directed thereto or to a different radio base station.
In accordance with one of the TDMA schemes, a common control signal is used by time division in the down-link radio signals transmitted from the radio base stations of the mobile communication network. In such an event, an overlap in time should not take place among down-link control slots in which the base radio stations transmit the down-link control signals. This is because a mobile station can not duly receive the down-link control slots while present alive in the overlapping area.
This undue reception of the down-link control slots can be avoided by transmission of a reference timing signal representative of common timing instants for reference from the control station to all radio base stations of the mobile communication network. Based on the reference timing signal, the radio base stations determine transmission time instants of their respective down-link control slots in individually predetermined manners of preventing the overlap in time from taking place.
It is, however, difficult in practice to use the abovedescribed manner of avoidance of the overlap in time when the radio base stations are controlled by two or more control stations. This is because the reference timing signal must be common to all radio base stations throughout the mobile communication network in spite of use of the control stations.
As consequence, it has been difficult in a conventional cellular TDMA mobile communication networks to reliably synchronize the down-link control slots transmitted from the radio base stations. This has resulted in an unstable service of the mobile communication network. Effective use of carriers for the down-link control signals have been impossible. In addition, it has been difficult to stably synchronize down-link communication slots used in transmitting the down-link communication data signals. This has made it difficult to effectively use frequency bands.